It is known to primarily use starch, dextrin, or casein glues for labeling glass containers. Starch glues represent simple adhesive systems and generally yield bonds with a good resistance to condensation. However, mechanical labeling requires the coordination of the criteria of numerous physical properties. Both during the processing stage and, lastly, on the finished labeled product, a number of demands are made on the adhesive. During processing the adhesive must be capable of flowing sufficiently and of having at the same time good adhesive properties, to be able to be processed on machines working with high-speed elements. Despite a sufficiently low viscosity, the wet adhesive should have a high initial adhesion, which prevents undesired displacement of a freshly attached label on a base, such as a bottle. The adhesion of the dried label is then tested by exposure of the labeled bottle to numerous stresses under the additional action of moisture, which the adhesion bond must withstand.
Mechanical labeling generally requires a certain viscosity range of from about 20,000 to 200,000 mPa.multidot.s, with ranges up to about 100,000 mPa.multidot.s, particularly from about 20,000 to 70,000 mPa.multidot.s, being suitable. In order not to exceed these viscosity ranges with starch glues, solids contents of 30% or less are usually used. Due to the resulting high water content, such starch glues do not usually have high initial adhesive power, which is particularly necessary in mechanical labeling on high-speed, automatic machines.
In comparison to starch glues, dextrin glues are generally more concentrated and thus have a higher initial adhesive power, so that they can also be used for labeling on high-speed, automatic machines. However, a disadvantage is their relatively low waterproofness, due to their relatively low molecular weight, so that they become highly brittle after drying and the labels either fall off or can be easily peeled off. They can not usually be used for labeling surface-tempered bottles.
The above-described disadvantages are not found in casein glues, which are frequently used for labeling. In comparison to starch and dextrin glues, the production of casein glues is relatively expensive, due to the cost of the raw material. In addition, casein, an aminal product, is subject to certain fluctuations in its properties, and thus at least partial replacement by other adhesive substances having positively adjustable and reproducible properties--and preferably being more economical than casein--would be desirable.